This application relates to compounds that inhibit heat shock protein 90 (Hsp90).
The Hsp90 family of proteins has four recognized members in mammalian cells; Hsp90 α and β, Grp94 and Trap-1. Hsp90 α and β exist in the cytosol and the nucleus in association with a number of other proteins. Hsp90 in its various form is the most abundant cellular chaperone, and has been shown in experimental systems to be required for ATP-dependent refolding of denatured or “unfolded” proteins. It has therefore been proposed to function as part of the cellular defense against stress. When cells are exposed to heat or other environmental stresses, the aggregation of unfolded proteins is prevented by pathways that catalyze their refolding or degradation. This process depends on the association of the unfolded protein in an ordered fashion with multiple chaperones (Hsp60, Hsp90, Hsp70 and p23), forming a “refoldosome” and ultimately the ATP-dependent release of the chaperones from the refolded protein.
Hsp90 can also play a role in maintaining the stability and function of mutated proteins. It seems to be required for expression of mutated p53 and v-src to a much greater extent than for their wild-type counterparts. It has been suggested that this occurs as a result of Hsp90-mediated suppression of the phenotypes of mutations that lead to protein unfolding.
Hsp90 is also necessary to the conformational maturation of several key proteins involved in the growth response of the cell to extracellular factors. These include the steroid receptors as well as certain kinases (i.e., Raf serine kinase, v-src and Her2). The mechanism whereby Hsp90 affects these proteins is not fully understood, but appears to be similar to its role in protein refolding. In the case of the progesterone receptor, it has been shown that binding and release of Hsp90 from the receptor occurs in a cyclic fashion in concert with release of other chaperones and immunophilins and is required for high affinity binding of the steroid to the receptor. Thus, Hsp90 could function as a physiologic regulator of signaling pathways, even in the absence of stress.
Hsp90 has been shown to be overexpressed in multiple tumor types and as a function of oncogenic transformation. Whether it plays a necessary role in maintaining transformation is unknown, but it could have at least three functions in this regard. Cancer cells grow in an environment of hypoxia, low pH and low nutrient concentration. They also rapidly adapt to or are selected to become resistant to radiation and cytotoxic chemotherapeutic agents. Thus, the general role of Hsp90 in maintaining the stability of proteins under stress may be necessary for cell viability under these conditions. Secondly, cancer cells harbor mutated oncogenic proteins. Some of these are gain-of-function mutations which are necessary far the transformed phenotype. Hsp90 may be required for maintaining the folded, functionally-active conformation of these proteins. Thirdly, activation of signaling pathways mediated by steroid receptors, Raf and other Hsp90 targets is necessary for the growth and survival of many tumors which thus probably also require functional Hsp90.
Hsp90 has been recognized as a viable target for therapeutic agents. Hsp90 family members possess a unique pocket in their N-terminal region that is specific to and conserved among all Hsp90s from bacteria to mammals, but which is not present in other molecular chaperones. The endogenous ligand for this pocket is not known, but it binds ATP and ADP with low affinity and has weak ATPase activity. The ansamycin antibiotics geldanamycin (GM) and herbimycin (HA) have been shown to bind to this conserved pocket, and this binding affinity has been shown for all members of the Hsp90 family. International Patent Publication No. WO98/51702 discloses the use of ansamycin antibiotics coupled to a targeting moiety to provide targeted delivery of the ansamycin leading to the degradation of proteins in and death of the targeted cells. International Patent Publication No. WO00/61578 relates to bifunctional molecules having two moieties which interact with the chaperone protein Hsp90, including in particular homo- and heterodimers of ansamycin antibiotics. These bifunctional molecules act to promote degradation and/or inhibition of HER-family tyrosine kinases and are effective for treatment of cancers which overexpress Her-kinases.
Exemplary small molecule therapeutics that bind to the same binding pocket of Hsp90 as ATP and the ansamycin antibiotics are disclosed in PCT Publication Nos. WO02/36075, WO2006/084030, WO2009/042646, WO2009/065035, and WO2011/044394; U.S. Pat. No. 7,834,181; and U.S. Patent Publication Nos. 2005/0113339, 2005/0004026, 2005/0049263, 2005/0256183, 2005/0119292, 2005/0113340, 2005/0107343, 2008/0096903, 2008/0234297, 2008/0234314, and 2009/0298857, all of which are incorporated herein by reference.
In particular, certain small molecule therapeutics that bind to the same binding pocket of Hsp90 can be described by the following general structural formula where Z1, Z2, and Z3 are selected from CH and N and the variable substituents are selected from a number of options:

While these compounds can be active as inhibitors of Hsp90, their level of activity is extremely variable with measured values for EC50 and IC50 being reported in anywhere from the micromolar to nanomolar ranges.